1. Field of the Invention
The present invention relates to power supplies and more particularly to a magnetic direct current to direct current converter having magnetic coupling feedback.
2. Description of the Prior Art
Converters typically switch an unidirectional source of current at a high frequency through transformer windings. An output winding of the transformer provides an alternating current which may be rectified to produce a d.c. source of power. A forward converter may include a switch which switches current through a transformer winding. An output winding provides the alternating current in response alternatively to build-up and collapse of flux of the transformer.
There are many electrical power applications with dc-to-dc converters requiring power to be supplied at a particular voltage or current level. Many power supplies convert the input power to output power at the desired current or voltage level. However, frequently, the input power is unregulated such that the output current or voltage varies. Also, the load conditions on the power supply may vary which in turn impacts the output current and/or voltage. Furthermore, the temperature at which the circuit is operating impacts the output current and/or voltage.
To maintain the power supply at a predetermined level, many power supplies have a regulator which monitors the output and modifies the input as necessary. Also, many applications require isolation of the output from the input. For example, it may be necessary to isolate the output side from noise existing on the input side. In switch-mode power supplies having a transformer, the input circuit is frequently electrically isolated from the output circuit, with a transistor switch in the input circuit tied to the primary coil and which is controlled by a feedback signal from the secondary coil. Electrical isolation between the primary and secondary coils may also be maintained by an opto-isolator which transmits the feedback information from the secondary coil side to the primary coil side. However, the linearity of opto-isolators commonly varies with temperature, with a dramatic decrease in gain at high temperature (above 85.degree. C.) and a permanent decrease over time. Also, the performance of opto-isolators is subject to radiation. The gain varies as a function of such radiation. Consequently, opto-isolators are generally precluded from military and satellite applications or other applications where the temperature varies significantly; long life is desired; and/or there are significant variations in radiation.
Magnetic isolation is a further form of isolation and uses a transformer or inductor sampling method of feedback. The benefits are relatively constant performance notwithstanding temperature and time variations. The disadvantages include complexity, low performance due to signal averaging, errors and noise feedback mixing with the desired signal.
The prior art includes U.S. Pat. No. 4,683,528, issued to Snow, et al. relating to a pulse position modulated regulation for power supplies. The circuitry includes a pulse transformer to transmit pulses from the secondary coil side of the transformer to the primary coil side with the power through the primary coil being controlled by varying the duty cycle of the drive signal for a transistor switch. U.S. Pat. No. 4,357,654, issued to Ikenoue, et al. discloses an inductively coupled dc-to-dc converter. The circuitry includes an inductance for storing energy when a switching transistor is turned on and releasing the stored energy when the switching transistor is turned off, and a semiconductor active element used as a flywheel element to provide the path for current resulting from the stored energy. U.S. Pat. No. 4,355,353, issued to Michael Farrer discloses a power supply apparatus having a transformer reset sensing circuit to inhibit reset of a switching transistor in the primary side of a transformer until resetting of the transformer.